Maximum Earth Fault Loop Impedance
The maximum earth fault loop impedance is used to calculate the available earth fault current within an installation connected at low voltage (below 1000V).
The following values are typical for low voltage installations:
| Earthing Type |
Maximum Earth Fault Loop Impedance |
| Protective Multiple Earthing (TN-C-S) |
0.35 ohms* |
| Cable Sheath Earth / Separate Continuous Earth Wire (TN-S) |
0.8 ohms* |
| Direct Earth (TT) |
Dependent on resistance of the customer’s earth electrode. Values exceeding 100 ohms can apply. |
* These values are sometimes exceeded where the connection is derived from a long overhead line network or from a small pole mounted transformer.